Single and double diaphragm forming are known techniques for making three dimensional components from a flat stack of composite material. During the diaphragm forming process the blank or composite stack is gradually shaped to take the form of an underlying forming tool whilst applying heat and pressure to the composite stack. Under the forming condition of applied heat and pressure interlaminar slip of the layers making up the composite stack allows the composite stack to take up the shape of the underlying tool with a minimum of fibre buckling or breakage. In double diaphragm forming a blank or composite stack is placed between two stretchable diaphragms of superplastic aluminium or polyimide film.
It is known to use double diaphragm forming with a solid male tool to produce composite parts that have a maximum of two radii of curvature. Such an example is illustrated in FIGS. 1A and 1B, wherein the solid male tool 1 has a substantially rectangular cross-section and the formed composite (as shown in FIG. 1B) is a channel 5 that is C-shaped.
FIG. 1A shows an example of a solid tool 1 with a flat stack of composite laminate 3 suspended above it. In the illustrated example, the composite material is suspended between two flexible diaphragms 3A, 3B to form a diaphragm pack. Air is evacuated from the diaphragm pack and it is heated before lowering the diaphragm pack onto the forming tool 1 such that it makes contact with the periphery of the tool. A vacuum is applied within the double diaphragm forming apparatus to the underside of the lower diaphragm 3A and the force of the atmospheric pressure bearing down on the upper diaphragm causes the diaphragm pack to grip on the periphery of the tool 1 so that the composite stack 3 is deformed to the shape of the tool 1 to produce a C-shaped channel 5 as illustrated in FIG. 1B.
In the example illustrated the forming tool includes two radii of curvature, each being provided between the horizontal face and the vertical faces of the forming tool.
U.S. Pat. No. 5,648,109 describes an example of using diaphragm forming to produce shapes of double curvature whilst preventing one of the diaphragms becoming trapped between the formed article and the forming tool. The diaphragm that could be trapped is withdrawn whilst the preform is brought into contact with the forming tool. Control of deformation of the preform into the shape of the forming tool may be achieved by using an inflatable member and utilises inflation and deflation of the members to allow the preform to move into contact with and take the shape of the forming tool in a controlled manner.